This invention relates to a metallized film capacitor used for in alternating current, and more particularly to such capacitor constructed with dry film capacitor bodies employing a vegetable oil as a coolant.
A wound or coiled capacitor body having a dielectric of polypropylene film with no liquid impregnant and using vapor-deposited electrodes on the film is sealed in a plastic or metal case. Such a capacitor unit is referred to as "dry film capacitor" and has been widely used in alternating current with normal current frequencies.
The application of peak voltages above 300 volts across this type of capacitor can result in partial discharge in the pocket of air trapped within film layers, causing localized damage to the film and the metallization. To avoid these disadvantages, thorough impregnation of an insulating oil throughout the wound body is essentially required.
However, thorough impregnation in a wound capacitor body has been difficult to achieve in a normal manufacturing process. Insulating gas impregnation in the wound body instead of insulating oil impregnation is available to conquer this difficulty. Moreover, using an electronegative gas, for example sulfur hexafluoride (SF6), as an insulating gas is more effective to gain a higher corona starting voltage than using common gases.
In general, the cooling effect in a gas medium is inferior to that in a liquid medium. Thus, the temperature rise in a wound body of a dry film capacitor of large power rating becomes too high to be acceptable, even though the dielectric loss of polypropylene is so slight.
Using vegetable oils as a coolant for polypropylene film wound capacitor bodies makes it possible to obtain effective cooling. Vegetable oils scarcely swell the polypropylene film, while insulating oils derived from petrochemical oils or synthetic hydrocarbon oils can easily swell the polypropylene film.
The swelling of the polypropylene film causes weakening of the end connection between the deposited electrode and the metal sprayed at the end, terminating in open circuits under severe conditions.
Vegetable oils mentioned above are needed to be dissolved with SF6 gas to the saturation point, after degassing the dissolved air in them, in order to prevent the gas diffusion from the wound body to oil.
In this invention, prior to loading a vegetable oil into the space between the wound bodies and the capacitor case, SF6 gas is introduced into the wound bodies to the appropriate degree of the gas pressure. Then the vegetable oil is prohibited from penetrating into the wound bodies by a stopping action due to the gas pressure.
The intentional rejection of the oil from thin gaps between film layers provides a preferable effect on clearing action of metallized film capacitors. The problems associated with clearing action by which the metallized electrode melts away from the point of breakthrough or short is deterioration of adjacent materials which in turn triggers further breakdown and tends to produce a catastrophic or avalanche result. A material such as vegetable oil is more subject to deteriorate by the clearing action than the material such as SF6
The dry wound capacitor with SF6 exhibits preferable results on the electric characteristics; corona starting voltage and breakdown voltage.
Along with these advantages, the total processing time of the gas impregnation and the coolant loading is comparatively less than is required for impregnation of the insulating oil by the prior art.